Digestions of materials in digesting liquids, such as strong acids, have been conducted in digesting systems wherein the digesting liquid was heated by microwave radiation. See, for example, European patent 0,416,759, European published specification 0,455,513 and U.S. Pat. No. 5,215,715. Extractions with solvents, wherein the solvent is sufficiently polar to absorb microwave radiation and convert it to heat, have also been employed as reported in Volume 371 of the Journal of Chromatography, at pages 299-306. A mixture of polar and non-polar solvents can be utilized, with the polar solvent absorbing microwave radiation and causing heating of the mixture.
Although digesting acids are corrosive, they are not usually explosive; however, many organic solvents are explosive when their vapors are mixed in a certain ratio with air. Thus, a safety problem can be encountered when conducting microwave heated extractions unless potential explosions are recognized and procedures are followed to prevent any such explosion. In U.S. Pat. No. 5,620,659 issued Apr. 15, 1997 in the name of Revesz, a system for preventing explosive incidents is disclosed. The system operates by detecting the presence of very small quantities of solvent vapors in exhaust air from a microwave chamber. When the solvent vapor concentration increases to explosive proportion, the source of microwave radiation, which otherwise could cause ignition of an explosive mixture of solvent vapor and air in the microwave chamber, is shut down. Alternatively the entire electrical system of the digesting apparatus is shut down in response to such detection. To be safe, such shutdowns occur when the content of solvent vapor in air is less than the lowest explosive limit. Thus, according to the teachings of Revesz, early detection of any leak in the system or of any rupture of a safety diaphragm on the extracting vessel is essential. The purpose of the rupture diaphragm is to prevent explosive failure of the extracting vessel due to excess pressure that might be developed within while the extracting solvent is heated.
European patent specification 0,455,513, the disclosure of which is hereby incorporated by reference, discloses such a rupture diaphragm in a digesting system. Further, U.S. Pat. No. 5,620,659 teaches the presence of a collecting container for collecting any vented vapor and liquid from a digesting container that has blown its rupture diaphragm. Though the system taught by Revesz is useful in preventing explosions, such a system is not useful in controlling an extraction process. From the teachings of Revesz, it is clear that once failure of an extracting vessel has occurred, monitoring for explosive chemicals is necessary. Commonly, when using microwave radiation for extraction, a plurality of vessels is placed within a microwave oven for simultaneous extraction. When a rupture diaphragm on a single vessel ruptures, the escaping solvent vapours may result in shutting down the system. Unfortunately, such a result affects all vessels and samples within the device. As such, it is not a desirable outcome. Further, when an extraction process is progressing, the rupturing of a rupture diaphragm results in an open topped vessel where a closed top vessel was desired. This open top often renders a resulting extraction unsuitable for analysis. Therefore, though a catastrophic failure such as an explosion is averted, it is only at the cost of the reactions in progress.
In U.S. Pat. No. 5,382,414 issued on Jan. 17, 1995 in the name of Lautenschlager and herein incorporated by reference, a resealable venting vessel cover is disclosed. Such a vessel cover obviates a need for a rupture disc by releasing gas from a vessel when pressure within the vessel is excessive and resealing the vessel when the pressure within the vessel is again below the threshold pressure. The released gas may accumulate in a collection chamber when present or in the microwave chamber itself when no collection chamber is used. The gas is vented through a venting system. The use of the resealable venting vessel cover, allows for continued extraction, digestion or hydrolysis of the sample contained within the vessel with only nominal loss of vessel contents.
In U.S. Pat. No. 5,447,077 issued Sep. 5, 1995 to Lautenschlager, which is hereby incorporated by reference, a device for microwave radiation heated evaporation is disclosed. Such a device employs vessels with outlets therein through which evaporated chemicals are discharged and an inlet through which air or other gases are fed into the vessels. It would be advantageous to provide a system for controlling a process heated by microwave radiation such as extraction, digestion or hydrolysis in order to improve the results of the process. It would also be advantageous to control a process heated by microwave radiation in order to avert explosive situations or potentially explosive situations without stopping the process.